These known compounds are used in numerous fields, in particular in medicine as radiopaque agents (contrast agents to X-rays) or as oxygen transporters in blood substitutes. A compound more particularly studied in this field is n-perfluorooctyl bromide C.sub.8 F.sub.17 Br.
Among the methods disclosed for preparation of these compounds, the following may be indicated more particularly:
the action of bromine on a compound R.sub.F -SF.sub.5 at 500.degree. C. in the presence of nickel (US Pat. No. 3,456,024);
the gas phase photolysis of a mixture of 1-hydrogenoperfluoroalkane and BrCl or BrF (Adcock et al., Chem. Abstr. 100 :34092e);
the action of bromine on compounds R.sub.F -I in the presence of a free radical initiator such as AIBN (Japanese Application Kokai No. 85-184033);
photobromination of the same iodated compounds by UV irradiation (Huang et al., Huaxue Xuebao 1984, 42 (10) 1106-8, summarised in Chem. Abstr. 102 : 78312x).
The poor yields obtained and/or the slow kinetics of these methods are the reason that they do not permit the economic production of perfluoroalkyl bromides on an industrial scale. In view of the importance of these compounds in the medical field, it is of greatest interest to be able to produce them at the lowest possible cost.
A process for the production of perfluoroalkyl bromides from the corresponding perfluoroalkanesulphonyl chlorides R.sub.F -SO.sub.2 Cl has been described in Patent Application No. EP 0298870. This process, which corresponds to the following reaction scheme: ##STR2## consists in reacting gaseous hydrogen bromide with a perfluoroalkanesulphonyl chloride in the presence of a catalyst consisting of an amine or a tertiary phosphine or a quaternary ammonium or phosphonium salt, at a temperature which can range from 80 to 200.degree. C. (preferably between 90 and 150.degree. C.); the amount of catalyst can range from 0.1 to 5 moles per 100 moles of chloride R.sub.F -SO.sub.2 Cl and is preferably between about 1 and 2 moles per 100. Although this process enables the perfluoroalkyl bromides to be produced in a single step, with an excellent yield and a very good selectivity, it requires the use of anhydrous gaseous HBr, a product which is expensive for a manufacturer who does not have a specific installation for the production of HBr available on his industrial site. On the other hand, in some cases, the reducing character of HBr can lead to the concomitant formation of sulphur-containing impurities (for example R.sub.F -S-S-R.sub.F and R.sub.F -SBr) and therefore to a drop in yield.